Multi-piece solid golf ball

ABSTRACT

A golf ball for amateur golfers is endowed with both an excellent flight when hit by golfers whose head speed is not very high and a soft yet good feel at impact. The ball has a core, an envelope layer, an intermediate layer and a cover. The core is formed primarily of a base rubber and the intermediate layer is formed so as to be thicker than the envelope layer and the cover. The respective surface hardnesses of the core, envelope layer-encased sphere, intermediate layer-encased sphere and ball satisfy a specific relationship. The core hardness profile is designed such that the surface areas A to F calculated from hardness differences between positions located at specific distances in the core and differences between the specific distances satisfy a specific formula.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2018-094643 filed in Japan on May 16, 2018, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to a multi-piece solid golf ball composed of four or more layers that include a core, an envelope layer, an intermediate layer and a cover.

BACKGROUND ART

Numerous innovations have hitherto been introduced in designing golf balls with a multilayer construction and many such balls have been developed to satisfy the needs of not only professional golfers and skilled amateurs, but also amateur golfers having mid or low head speeds. For example, functional multi-piece solid golf balls in which the surface hardnesses of the respective layers—i.e., the core, envelope layer, intermediate layer and cover (outermost layer)—have been optimized are widely used.

Examples of such multi-piece solid golf balls include those disclosed in the following patent publications: JP-A 2014-132955, JP-A 2015-173860, JP-A 2016-16117 and JP-A 2016-179052. These publicly disclosed golf balls satisfy the following hardness relationship among the layers: surface hardness of ball >surface hardness of intermediate layer >surface hardness of envelope layer <surface hardness of core, and impart an excellent flight performance even when played by amateur golfers who do not have a high head speed.

However, no effort has been made in these prior-art golf balls to optimize the hardness profile of the core and the thickness relationship among the layers. Hence, when used as manufactured balls targeted at low-head-speed golfers, there remains room for improvement in obtaining an even more improved flight performance and a good feel at impact.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a multi-piece solid golf ball for amateur golfers which has an excellent flight when hit by golfers whose head speed is not that high and which also has a soft yet good feel at impact.

As a result of extensive investigations, we have discovered that, in a multi-piece solid golf ball having a core, an envelope layer, an intermediate layer and a cover, by specifying the relationships among the thicknesses of these layers and the relationships among their surface hardnesses and by designing the core hardness profile such that, setting the hardness values at positions located specific distances from a midpoint M between the center and surface of the core toward the surface side of the core and the hardness values at positions located specific distances from the midpoint M toward the center side of the core and calculating in the manner described below surface areas A to F from hardness differences between the positions and differences between the specific distances, these surface areas A to F satisfy a specific formula, golfers whose head speeds are not very high are able to achieve a good flight performance and a soft yet solid feel at impact can be obtained.

Accordingly, the invention provides a multi-piece solid golf ball which has a core, an envelope layer, an intermediate layer and a cover, wherein the core is formed primarily of a base rubber and the intermediate layer is formed so as to be thicker than the envelope layer and the cover. The surface hardness of the core, the surface hardness of the sphere obtained by encasing the core with the envelope layer (envelope layer-encased sphere), the surface hardness of the sphere obtained by encasing the envelope layer-encased sphere with the intermediate layer (intermediate layer-encased sphere) and the surface hardness of the ball together satisfy the following relationship:

-   -   surface hardness of ball >surface hardness of intermediate         layer-encased sphere >surface hardness of envelope layer-encased         sphere <surface hardness of core.

The core has a hardness profile in which, letting Cc be the Shore C hardness at a center of the core, Cs be the Shore C hardness at the core surface, C_(M) be the Shore C hardness at a midpoint M between the core center and surface, C_(M+2.5), C_(M+5.0) and C_(M+7.5) be the Shore C hardnesses at, respectively, positions 2.5 mm, 5.0 mm and 7.5 mm from the midpoint M toward the core surface side, and C_(M−2.5), C_(M−5.0) and C_(M−7.5) be the Shore C hardnesses at, respectively, positions 2.5 mm, 5.0 mm and 7.5 mm from the midpoint M toward the core center side, the surface areas A to F defined as follows

-   -   surface area A: ½×2.5×(C_(M−5.0)−C_(M−7.5)),     -   surface area B: ½×2.5×(C_(M−2.5)−C_(M−5.0)),     -   surface area C: ½×2.5×(C_(M)−C_(M−2.5)),     -   surface area D: ½×2.5×(C_(M+2.5)−C_(M)),     -   surface area E: ½×2.5×(C_(M+5.0)−C_(M+2.5)),     -   surface area F: ½×2.5×(C_(M+7.5)−C_(M+5.0)),         satisfy the condition

(surface area D+surface area E+surface area F)−(surface area A+surface area B+surface area C)>0.

In a preferred embodiment of the golf ball of the invention, the surface areas A to F in the core hardness profile satisfy the condition

(surface area D+surface area E)−(surface area A+surface area B+surface area C)≥0.

In another preferred embodiment, the hardness difference between the surface and center of the core (Cs−Cc), expressed in terms of Shore C hardness, is at least 20.

In yet another preferred embodiment, the surface areas A to F in the core hardness profile satisfy the condition

0.15≤[(surface area D+surface area E+surface area F)−(surface area A+surface area B+surface area C)]/(Cs−Cc)≤0.6.

In still another preferred embodiment, the cover has a thickness of 1.2 mm or less.

In a further preferred embodiment, the cover has a plurality of dimples formed on a surface thereof, the ball has arranged thereon at least one dimple with a cross-sectional shape that is described by a curved line or a combination of straight and curved lines and specified by steps (i) to (iv) below, and the total number of dimples is from 250 to 380:

(i) letting the foot of a perpendicular drawn from a deepest point of the dimple to an imaginary plane defined by a peripheral edge of the dimple be the dimple center and a straight line that passes through the dimple center and any one point on the edge of the dimple be the reference line;

(ii) dividing a segment of the reference line from the dimple edge to the dimple center into at least 100 points and computing the distance ratio for each point when the distance from the dimple edge to the dimple center is set to 100%;

(iii) computing the dimple depth ratio at every 20% from 0 to 100% of the distance from the dimple edge to the dimple center; and

(iv) at the depth ratios in dimple regions 20 to 100% of the distance from the dimple edge to the dimple center, determining the change in depth ΔH every 20% of said distance and designing a dimple cross-sectional shape such that the change ΔH is at least 6% and not more than 24% in all regions corresponding to from 20 to 100% of said distance.

In a still further preferred embodiment, the cover has a paint film layer formed on a surface thereof, which paint film layer has a hardness on the Shore C scale of from 40 to 80.

Advantageous Effects of the Invention

The multi-piece solid golf ball of the invention has an excellent flight performance when hit by golfers whose head speeds are not that high and also has a soft yet good feel at impact, making it highly suitable for use by amateur golfers.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a schematic cross-sectional view of a multi-piece solid golf ball according to one embodiment of the invention.

FIG. 2 is a graph that uses core hardness profile data from Working Example 1 to explain surface areas A to F in a core hardness profile.

FIG. 3A and FIG. 3B present schematic cross-sectional views of dimples used in the Working Examples and Comparative Examples, FIG. 3A showing a dimple having a distinctive cross-sectional shape and FIG. 3B showing a dimple having a circularly arcuate cross-sectional shape.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the appended diagrams.

The multi-piece solid golf ball of the invention has a core, an envelope layer, an intermediate layer and a cover. Referring to FIG. 1, which shows an embodiment of the inventive golf ball, the ball G has a core 1, an envelope layer 2 encasing the core 1, an intermediate layer 3 encasing the envelope layer 2, and a cover 4 encasing the intermediate layer 3. The cover 4, excluding a paint film layer, is positioned as the outermost layer in the layered construction of the ball. In this invention, the intermediate layer and the envelope layer may be single layers or may be formed of two or more layers. Numerous dimples D are typically formed on the surface of the cover (outermost layer) 4 so as to enhance the aerodynamic properties of the ball. A paint film layer 5 is formed on the surface of the cover 4. Each layer is described in detail below.

The core has a diameter of preferably at least 35.3 mm, more preferably at least 35.6 mm, and even more preferably at least 36.0 mm. The upper limit is preferably not more than 37.5 mm, more preferably not more than 37.0 mm, and even more preferably not more than 36.5 mm. When the core diameter is too small, the spin rate on shots with a driver (W#1) may become high, as a result of which it may not be possible to achieve the desired distance. On the other hand, when the core diameter is too large, the durability to repeated impact may worsen and the feel at impact may worsen.

The core has a deflection (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) which, although not particularly limited, is preferably at least 3.0 mm, more preferably at least 3.5 mm, and even more preferably at least 4.0 mm. The upper limit is preferably not more than 7.0 mm, more preferably not more than 6.0 mm, and even more preferably not more than 5.0 mm. When the core deflection is too small, i.e., when the core is too hard, the spin rate of the ball may rise excessively and a good distance may not be achieved, or the feel at impact may be too hard. On the other hand, when the core deflection is too large, i.e., when the core is too soft, the ball rebound may become too low and a good distance may not be achieved, the feel at impact may be too soft, or the durability to cracking on repeated impact may worsen.

The core material is composed primarily of a rubber material. Specifically, a core-forming rubber composition can be prepared by using a base rubber as the chief component and including, together with this, other ingredients such as a co-crosslinking agent, an organic peroxide, an inert filler and an organosulfur compound. It is preferable to use polybutadiene as the base rubber.

Commercial products may be used as the polybutadiene. Illustrative examples include BR01, BR51 and BR730 (from JSR Corporation). The proportion of polybutadiene within the base rubber is preferably at least 60 wt %, and more preferably at least 80 wt %. Rubber ingredients other than the above polybutadienes may be included in the base rubber, provided that doing so does not detract from the advantageous effects of the invention. Examples of rubber ingredients other than the above polybutadienes include other polybutadienes and also other diene rubbers, such as styrene-butadiene rubbers, natural rubbers, isoprene rubbers and ethylene-propylene-diene rubbers.

Examples of co-crosslinking agents include unsaturated carboxylic acids and the metal salts of unsaturated carboxylic acids. Specific examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid and fumaric acid. The use of acrylic acid or methacrylic acid is especially preferred. Metal salts of unsaturated carboxylic acids include, without particular limitation, the above unsaturated carboxylic acids that have been neutralized with desired metal ions. Specific examples include the zinc salts and magnesium salts of methacrylic acid and acrylic acid. The use of zinc acrylate is especially preferred.

The unsaturated carboxylic acid and/or metal salt thereof is included in an amount, per 100 parts by weight of the base rubber, which is typically at least 5 parts by weight, preferably at least 9 parts by weight, and more preferably at least 13 parts by weight. The amount included is typically not more than 60 parts by weight, preferably not more than 50 parts by weight, and more preferably not more than 40 parts by weight. Too much may make the core too hard, giving the ball an unpleasant feel at impact, whereas too little may lower the rebound.

Commercial products may be used as the organic peroxide. Examples of such products that may be suitably used include Percumyl D, Perhexa C-40 and Perhexa 3M (all from NOF Corporation), and Luperco 231XL (from AtoChem Co.). One of these may be used alone, or two or more may be used together. The amount of organic peroxide included per 100 parts by weight of the base rubber is preferably at least 0.1 part by weight, more preferably at least 0.3 part by weight, even more preferably at least 0.5 part by weight, and most preferably at least 0.6 part by weight. The upper limit is preferably not more than 5 parts by weight, more preferably not more than 4 parts by weight, even more preferably not more than 3 parts by weight, and most preferably not more than 2.5 parts by weight. When too much or too little is included, it may not be possible to obtain a ball having a good feel, durability and rebound.

Another compounding ingredient typically included with the base rubber is an inert filler, preferred examples of which include zinc oxide, barium sulfate and calcium carbonate. One of these may be used alone, or two or more may be used together. The amount of inert filler included per 100 parts by weight of the base rubber is preferably at least 1 part by weight, and more preferably at least 5 parts by weight. The upper limit is preferably not more than 50 parts by weight, more preferably not more than 40 parts by weight, and even more preferably not more than 35 parts by weight. Too much or too little inert filler may make it impossible to obtain a proper weight and a suitable rebound.

In addition, an antioxidant may be optionally included. Illustrative examples of suitable commercial antioxidants include Nocrac NS-6 and Nocrac NS-30 (both available from Ouchi Shinko Chemical Industry Co., Ltd.), and Yoshinox 425 (available from Yoshitomi Pharmaceutical Industries, Ltd.). One of these may be used alone, or two or more may be used together.

The amount of antioxidant included per 100 parts by weight of the base rubber is set to preferably 0 part by weight or more, more preferably at least 0.05 part by weight, and even more preferably at least 0.1 part by weight. The upper limit is set to preferably not more than 3 parts by weight, more preferably not more than 2 parts by weight, even more preferably not more than 1 part by weight, and most preferably not more than 0.5 part by weight. Too much or too little antioxidant may make it impossible to achieve a suitable ball rebound and durability.

An organosulfur compound may be included in the core in order to impart a good resilience. The organosulfur compound is not particularly limited, provided it can enhance the rebound of the golf ball. Exemplary organosulfur compounds include thiophenols, thionaphthols, halogenated thiophenols, and metal salts of these. Specific examples include pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, p-chlorothiophenol, the zinc salt of pentachlorothiophenol, the zinc salt of pentafluorothiophenol, the zinc salt of pentabromothiophenol, the zinc salt of p-chlorothiophenol, and any of the following having 2 to 4 sulfur atoms: diphenylpolysulfides, dibenzylpolysulfides, dibenzoylpolysulfides, dibenzothiazoylpolysulfides and dithiobenzoylpolysulfides. The use of the zinc salt of pentachlorothiophenol is especially preferred.

It is recommended that the amount of organosulfur compound included per 100 parts by weight of the base rubber be preferably 0 part by weight or more, more preferably at least 0.05 part by weight, and even more preferably at least 0.1 part by weight, and that the upper limit be preferably not more than 5 parts by weight, more preferably not more than 3 parts by weight, and even more preferably not more than 2.5 parts by weight. Including too much organosulfur compound may make a greater rebound-improving effect (particularly on shots with a W#1) unlikely to be obtained, may make the core too soft or may worsen the feel of the ball at impact. On the other hand, including too little may make a rebound-improving effect unlikely.

More specifically, decomposition of the organic peroxide within the core formulation can be promoted by the direct addition of water (or a water-containing material) to the core material. The decomposition efficiency of the organic peroxide within the core-forming rubber composition is known to change with temperature; starting at a given temperature, the decomposition efficiency rises with increasing temperature. If the temperature is too high, the amount of decomposed radicals rises excessively, leading to recombination between radicals and, ultimately, deactivation. As a result, fewer radicals act effectively in crosslinking. Here, when a heat of decomposition is generated by decomposition of the organic peroxide at the time of core vulcanization, the vicinity of the core surface remains at substantially the same temperature as the temperature of the vulcanization mold, but the temperature near the core center, due to the build-up of heat of decomposition by the organic peroxide which has decomposed from the outside, becomes considerably higher than the mold temperature. In cases where water (or a water-containing material) is added directly to the core, because the water acts to promote decomposition of the organic peroxide, radical reactions like those described above can be made to differ at the core center and core surface. That is, decomposition of the organic peroxide is further promoted near the center of the core, bringing about greater radical deactivation, which leads to a further decrease in the amount of active radicals. As a result, it is possible to obtain a core in which the crosslink densities at the core center and the core surface differ markedly. It is also possible to obtain a core having different dynamic viscoelastic properties at the core center.

The water included in the core material is not particularly limited, and may be distilled water or tap water. The use of distilled water that is free of impurities is especially preferred. The amount of water included per 100 parts by weight of the base rubber is preferably at least 0.1 part by weight, and more preferably at least 0.3 parts by weight. The upper limit is preferably not more than 5 parts by weight, and more preferably not more than 4 parts by weight.

The core can be produced by vulcanizing and curing the rubber composition containing the above ingredients. For example, the core can be produced by using a Banbury mixer, roll mill or other mixing apparatus to intensively mix the rubber composition, subsequently compression molding or injection molding the mixture in a core mold, and curing the resulting molded body by suitably heating it under conditions sufficient to allow the organic peroxide or co-crosslinking agent to act, such as at a temperature of between 100 and 200° C., preferably between 140 and 180° C., for 10 to 40 minutes.

The core may consist of a single layer alone, or may be formed as a two-layer core consisting of an inner core layer and an outer core layer. When the core is formed as a two-layer core consisting of an inner core layer and an outer core layer, the inner core layer and outer core layer materials may each be composed primarily of the above-described rubber material. Also, the rubber material making up the outer core layer encasing the inner core layer may be the same as or different from the inner core layer material. The details here are the same as those given above for the ingredients of the core-forming rubber material.

Next, the hardness profile of the core is described. The core hardness described below refers to the Shore C hardness. This Shore C hardness is the hardness value measured with a Shore C durometer in general accordance with ASTM D2240. Although, for example, the timing of the read-off of measurements differs from that in the technique used for measuring JIS-C hardness, the measured Shore C hardness values do not differ much from and, in fact, are closely similar to the JIS-C values.

The hardness at the core center (Cc) is preferably at least 50, more preferably at least 52, and even more preferably at least 54. The upper limit is preferably not more than 59, more preferably not more than 57, and even more preferably not more than 55. When this value is too large, the feel at impact may become hard, or the spin rate may rise on full shots, as a result of which the intended distance may not be achieved. On the other hand, when this value is too small, the rebound may become low, resulting in a poor distance, or the durability to cracking on repeated impact may worsen.

The hardness at a position 2.5 mm from the core center (C2.5) is preferably at least 51, more preferably at least 53, and even more preferably at least 55. The upper limit is preferably not more than 61, more preferably not more than 59, and even more preferably not more than 57. A value outside of this range may lead to undesirable results similar to those described above for the core center hardness (Cc).

The hardness at a position 5 mm from the core center (C5) is preferably at least 54, more preferably at least 56, and even more preferably at least 58. The upper limit is preferably not more than 63, more preferably not more than 61, and even more preferably not more than 59. A value outside of this range may lead to undesirable results similar to those described above for the core center hardness (Cc).

The hardness at a position 7.5 mm from the core center (C7.5) is preferably at least 56, more preferably at least 58, and even more preferably at least 60. The upper limit is preferably not more than 65, more preferably not more than 63, and even more preferably not more than 61. A value outside of this range may lead to undesirable results similar to those described above for the core center hardness (Cc).

The hardness at a position 10 mm from the core center (C10) is preferably at least 59, more preferably at least 61, and even more preferably at least 63. The upper limit is preferably not more than 68, more preferably not more than 66, and even more preferably not more than 64. A value outside of this range may lead to undesirable results similar to those described above for the core center hardness (Cc).

The hardness at a position 12.5 mm from the core center (C12.5) is preferably at least 64, more preferably at least 66, and even more preferably at least 68. The upper limit is preferably not more than 75, more preferably not more than 73, and even more preferably not more than 71. A value outside of this range may lead to undesirable results similar to those described above for the core center hardness (Cc).

The hardness at a position 15 mm from the core center (C15) is preferably at least 69, more preferably at least 71, and even more preferably at least 73. The upper limit is preferably not more than 81, more preferably not more than 79, and even more preferably not more than 77. A value outside of this range may lead to undesirable results similar to those described above for the core center hardness (Cc).

The hardness at the core surface (Cs) is preferably at least 73, more preferably at least 75, and even more preferably at least 77. The upper limit is preferably not more than 85, more preferably not more than 83, and even more preferably not more than 81. A value outside of this range may lead to undesirable results similar to those described above for the core center hardness (Cc).

The difference between the core surface hardness (Cs) and the core center hardness (Cc) is preferably at least 20, more preferably at least 22, and even more preferably at least 24. The upper limit is preferably not more than 35, more preferably not more than 32, and even more preferably not more than 28. When this value is too small, the ball spin rate-lowering effect on shots with a driver may be inadequate, resulting in a poor distance. When this value is too large, the initial velocity of the ball when struck may decrease, resulting in a poor distance, or the durability to cracking on repeated impact may worsen.

In the above core hardness profile in this invention, letting Cc be the Shore C hardness at the core center, Cs be the Shore C hardness at the core surface, C_(M) be the Shore C hardness at a midpoint M between the core center and surface, C_(M+2.5), C_(M+5.0) and C_(M+7.5) be the Shore C hardnesses at, respectively, positions 2.5 mm, 5.0 mm and 7.5 mm from the midpoint M toward the core surface side, and C_(M−2.5), C_(M−5.0) and C_(M−7.5) be the Shore C hardnesses at, respectively, positions 2.5 mm, 5.0 mm and 7.5 mm from the midpoint M toward the core center side,

the surface areas A to F defined as follows

-   -   surface area A: ½×2.5×(C_(M−5.0)−C_(M−7.5)),     -   surface area B: ½×2.5×(C_(M−2.5)−C_(M−5.0)),     -   surface area C: ½×2.5×(C_(M)−C_(M−2.5)),     -   surface area D: ½×2.5×(C_(M+2.5)−C_(M)),     -   surface area E: ½×2.5×(C_(M+5.0)−C_(M+2.5)),     -   surface area F: ½×2.5×(C_(M+7.5)−C_(M+5.0)),         satisfy the condition (surface area D+surface area E+surface         area F)−(surface area A+surface area B+surface area C)>0. That         is, the core hardness profile is characterized in that the value         (surface area D+surface area E+surface area F) is larger than         the value (surface hardness A+surface hardness B+surface         hardness C). FIG. 2 shows a graph that uses core hardness         profile data from Working Example 1 to explain surface areas A         to F. As is apparent from the graph, each of surface areas A to         F is the surface area of a triangle whose base is the difference         between specific distances and whose height is the difference in         hardness between the positions at these specific distances.

The lower limit value of (surface area D+surface area E+surface area F)−(surface area A+surface area B+surface area C) above is greater than 0, preferably 3 or more, and more preferably 6 or more. This value has no particular upper limit, although it is preferably not more than 20, more preferably not more than 15, and even more preferably not more than 10. When this value is too small, the spin rate-lowering effect on shots with a driver (W#1) may be inadequate and a good distance may not be achieved. On the other hand, when this value is too large, the initial velocity of the ball when struck may be low and a good distance may not be achieved, or the durability to cracking on repeated impact may worsen.

In the core hardness profile, it is preferable for the following condition to be satisfied: 0.15≤[(surface area D+surface area E+surface area F)−(surface area A+surface area B+surface area C)]/(Cs−Cc)≤0.6. This value has a lower limit of preferably at least 0.20, and more preferably at least 0.25. There is no particular need to set an upper limit for the value in this formula, although this value is preferably not more than 0.60, more preferably not more than 0.50, and even more preferably not more than 0.40. When this value is too small, the spin rate-lowering effect on shots with a driver (W#1) may be inadequate and so a good distance may not be achieved. On the other hand, when this value is too large, the initial velocity of the ball when struck may be low, resulting in a poor distance, or the durability to cracking on repeated impact may worsen.

In addition, in the core hardness profile, it is preferable for the following condition to be satisfied: (surface area D+surface area E)−(surface area A+surface area B+surface area C)≥0. This value has a lower limit of preferably at least 0.5, and more preferably at least 1.0. There is no particular need to set an upper limit for this value, although it is preferably not more than 8.0, more preferably not more than 6.0, and even more preferably not more than 4.0. When this value is too small, the spin rate-lowering effect on shots with a driver (W#1) may be inadequate and a good distance may not be obtained. On the other hand, when this value is too large, the initial velocity of the ball when struck may be low, resulting in a poor distance, or the durability to cracking on repeated impact may worsen.

Next, the envelope layer is described.

The envelope layer has a material hardness on the Shore D scale which, although not particularly limited, is preferably at least 15, more preferably at least 20, and even more preferably at least 25. The upper limit is preferably not more than 45, more preferably not more than 40, and even more preferably not more than 30. The surface hardness of the sphere obtained by encasing the core with the envelope layer (envelope layer-encased sphere), expressed on the Shore D scale, is preferably at least 23, more preferably at least 28, and even more preferably at least 33. The upper limit is preferably not more than 53, more preferably not more than 48, and even more preferably not more than 38. When the material and surface hardnesses of the envelope layer are lower than the above respective ranges, the spin rate of the ball on full shots may rise excessively, resulting in a poor distance, or the durability of the ball to repeated impact may worsen. On the other hand, when the material and surface hardnesses are too high, the durability to cracking on repeated impact may worsen or the spin rate on full shots may rise, as a result of which, particularly on low head speed shots, a good distance may not be achieved, and the feel at impact may worsen.

The envelope layer has a thickness of preferably at least 0.5 mm, more preferably at least 0.7 mm, and even more preferably at least 0.9 mm. The upper limit in the envelope layer thickness is preferably not more than 1.4 mm, more preferably not more than 1.2 mm, and even more preferably not more than 1.0 mm. When this envelope layer is too thin, the durability to cracking on repeated impact may worsen or the feel at impact may worsen. When the envelope layer is too thick, the spin rate of the ball on full shots may rise and a good distance may not be obtained.

The envelope layer material is not particularly limited, although various types of thermoplastic resin materials may be suitably employed for this purpose. For example, use can be made of ionomer resins, urethane, amide, ester, olefin or styrene-type thermoplastic elastomers, and mixtures thereof. From the standpoint of obtaining a good rebound in the desired hardness range, the use of a thermoplastic polyether ester elastomer is especially suitable.

The sphere obtained by encasing the core with the envelope layer (envelope layer-encased sphere) has a deflection (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) which, although not particularly limited, is preferably at least 3.0 mm, more preferably at least 3.5 mm, and even more preferably at least 4.0 mm. The upper limit is preferably not more than 7.0 mm, more preferably not more than 6.0 mm, and even more preferably not more than 5.0 mm. When the sphere deflection is too small, that is, when the sphere is too hard, the ball spin rate may rise excessively, resulting in a poor distance, or the feel at impact may become too hard. On the other hand, when the sphere deflection is too large, that is, when the sphere is too soft, the ball rebound may become too low, resulting in a poor distance, the feel at impact may become too soft, or the durability to cracking on repeated impact may worsen.

Next, the intermediate layer is described.

The intermediate layer has a material hardness on the Shore D scale which, although not particularly limited, is preferably at least 40, more preferably at least 45, and even more preferably at least 47. The upper limit is preferably not more than 60, more preferably not more than 55, and even more preferably not more than 53. The surface hardness of the sphere obtained by encasing the envelope layer-encased sphere with the intermediate layer (intermediate layer-encased sphere), expressed on the Shore D scale, is preferably at least 46, more preferably at least 51, and even more preferably at least 53. The upper limit is preferably not more than 66, more preferably not more than 61, and even more preferably not more than 59. When the material and surface hardnesses of the intermediate layer are lower than the above respective ranges, the spin rate of the ball on full shots may rise excessively, resulting in a poor distance, or the durability of the ball to repeated impact may worsen. On the other hand, when the material and surface hardnesses are too high, the durability to cracking on repeated impact may worsen or the spin rate on full shots may rise, as a result of which, particularly on low head speed shots, a good distance may not be achieved, and the feel at impact may worsen.

The intermediate layer has a thickness of preferably at least 0.7 mm, more preferably at least 0.9 mm, and even more preferably at least 1.1 mm. The upper limit in the intermediate layer thickness is preferably not more than 1.5 mm, more preferably not more than 1.3 mm, and even more preferably not more than 1.2 mm. When the intermediate layer is too thin, the durability to cracking on repeated impact may worsen or the feel at impact may worsen. When the intermediate layer is too thick, the spin rate of the ball on full shots may rise and a good distance may not be obtained.

The intermediate layer-forming material is not particularly limited and may be a known resin. Examples of preferred materials include resin compositions containing as the essential ingredients:

100 parts by weight of a resin component composed of, in admixture,

(A) a base resin of (a-1) an olefin-unsaturated carboxylic acid random copolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer mixed with (a-2) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer in a weight ratio between 100:0 and 0:100, and

(B) a non-ionomeric thermoplastic elastomer in a weight ratio between 100:0 and 50:50;

(C) from 5 to 80 parts by weight of a fatty acid and/or fatty acid derivative having a molecular weight of from 228 to 1,500; and

(D) from 0.1 to 17 parts by weight of a basic inorganic metal compound capable of neutralizing un-neutralized acid groups in components (A) and (C).

Components (A) to (D) in the intermediate layer-forming resin material described in, for example, JP-A 2010-253268 may be advantageously used as above components (A) to (D).

A non-ionomeric thermoplastic elastomer may be included in the intermediate layer material. The amount of non-ionomeric thermoplastic elastomer included is preferably from 0 to 50 parts by weight per 100 parts by weight of the total amount of the base resin.

Exemplary non-ionomeric thermoplastic elastomers include polyolefin elastomers (including polyolefin and metallocene polyolefins), polystyrene elastomers, diene polymers, polyacrylate polymers, polyamide elastomers, polyurethane elastomers, polyester elastomers and polyacetals.

Depending on the intended use, optional additives may be suitably included in the intermediate layer material. For example, pigments, dispersants, antioxidants, ultraviolet absorbers and light stabilizers may be added. When these additives are included, the amount added per 100 parts by weight of the overall base resin is preferably at least 0.1 part by weight, and more preferably at least 0.5 part by weight. The upper limit is preferably not more than 10 parts by weight, and more preferably not more than 4 parts by weight.

The sphere obtained by encasing the envelope-encased sphere with the intermediate layer (intermediate layer-encased sphere) has a deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) which, although not particularly limited, is preferably at least 2.7 mm, more preferably at least 3.2 mm, and even more preferably at least 3.7 mm. The upper limit is preferably not more than 6.7 mm, more preferably not more than 5.7 mm, and even more preferably not more than 4.7 mm. When the sphere deflection is too small, that is, when the sphere is too hard, the ball spin rate may rise excessively, resulting in a poor distance, or the feel at impact may become too hard. On the other hand, when the sphere deflection is too large, that is, when the sphere is too soft, the ball rebound may become too low, resulting in a poor distance, the feel at impact may become too soft, or the durability to cracking on repeated impact may worsen.

Next, the cover is described.

The cover has a material hardness on the Shore D scale which, although not particularly limited, is preferably at least 55, more preferably at least 59, and even more preferably at least 61. The upper limit is preferably not more than 70, more preferably not more than 68, and even more preferably not more than 65. The surface hardness of the sphere obtained by encasing the intermediate layer-encased sphere with the cover (ball), expressed on the Shore D scale, is preferably at least 61, more preferably at least 65, and even more preferably at least 67. The upper limit is preferably not more than 76, more preferably not more than 74, and even more preferably not more than 71. When the material hardness of the cover and the ball surface hardness are too much lower than the above respective ranges, the spin rate of the ball on shots with a driver (W#1) may rise and the ball initial velocity may decrease, as a result of which a good distance may not be obtained. On the other hand, when the material hardness of the cover and the ball surface hardness are too high, the durability to cracking on repeated impact may worsen.

The cover has a thickness of preferably at least 0.6 mm, more preferably at least 0.8 mm, and even more preferably at least 1.0 mm. The upper limit in the cover thickness is preferably not more than 1.2 mm, more preferably not more than 1.15 mm, and even more preferably not more than 1.1 mm. When the cover is too thin, the durability to cracking on repeated impact may worsen. When the cover is too thick, the spin rate of the ball on shots with a driver (W#1) may rise and a good distance may not be obtained, or the feel at impact in the short game and on shots with a putter may be too hard.

Various types of thermoplastic resins, particularly ionomer resins, that are employed as cover stock in golf balls may be suitably used as the cover material in this invention. Commercial products may be used as the ionomer resin. Alternatively, the cover-forming resin material that is used may be one obtained by blending, of commercially available ionomer resins, a high-acid ionomer resin having an acid content of at least 18 wt % into a conventional ionomer resin. The high rebound and spin rate-lowering effect obtained with such a blend makes it possible to achieve a good distance on shots with a driver (W#1). The amount of such a high-acid ionomer resin included per 100 parts by weight of the resin material is preferably at least 10 wt %, more preferably at least 30 wt %, and even more preferably at least 60 wt %. The upper limit is generally up to 100 wt %, preferably up to 90 wt %, and more preferably up to 80 wt %. When the content of this high-acid ionomer resin is too low, the spin rate on shots with a driver (W#1) may rise, resulting in a poor distance. On the other hand, when the content of the high-acid ionomer resin is too high, the durability to cracking on repeated impact may worsen.

The sphere obtained by encasing the intermediate layer-encased sphere with the cover (i.e., the ball) has a deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) which, although not particularly limited, is preferably at least 2.4 mm, more preferably at least 2.9 mm, and even more preferably at least 3.4 mm. The upper limit is preferably not more than 5.0 mm, more preferably not more than 4.5 mm, and even more preferably not more than 4.0 mm. When the ball deflection is too small, i.e., when the ball is too hard, the spin rate of the ball may rise excessively and a good distance may not be achieved, or the feel at impact may be too hard. On the other hand, when the deflection of the ball is too large, i.e., when the ball is too soft, the ball rebound may become too low, resulting in a poor distance, the feel at impact may be too soft, or the durability to cracking on repeated impact may worsen.

The manufacture of multi-piece solid golf balls in which the above-described core, envelope layer, intermediate layer and cover (outermost layer) are formed as successive layers may be carried out by a customary method such as a known injection molding process. For example, a multi-piece golf ball can be produced by successively injection-molding the envelope layer material and the intermediate layer material over the core so as to obtain an intermediate layer-encased sphere, and then injection-molding the cover material over the intermediate layer-encased sphere. Alternatively, the encasing layers may each be formed by enclosing the sphere to be encased within two half-cups that have been pre-molded into hemispherical shapes and then molding under applied heat and pressure.

Hardness Relationship Among Layers

In this invention, it is critical for the hardness relationship among the layers to satisfy the following condition: surface hardness of ball >surface hardness of intermediate layer-encased sphere >surface hardness of envelope layer-encased sphere <surface hardness of core. When this hardness relationship is not satisfied, a good flight performance and a feel on impact that is both soft and also solid may not be obtained at both mid and low head speeds.

As indicated above, the surface hardness of the ball must be higher than the surface hardness of the intermediate layer-encased sphere. The difference between this ball surface hardness and the intermediate layer surface hardness, expressed on the Shore D scale, is preferably from 4 to 20, more preferably from 6 to 17, and even more preferably from 8 to 13. When this difference is small, the spin rate-lowering effect on full shots may be inadequate, as a result of which a good distance may not be achieved. On the other hand, when this difference is too large, the durability to cracking on repeated impact may worsen.

As indicated above, the intermediate layer-encased sphere must have a higher surface hardness than the envelope layer-encased sphere. The difference between the surface hardness of the intermediate layer-encased sphere and the surface hardness of the envelope layer-encased sphere, expressed on the Shore D scale, is preferably from 4 to 40, more preferably from 6 to 30, and even more preferably from 10 to 23. When this difference is small, it may not be possible to obtain a feel at impact that is both soft and solid. On the other hand, when this difference is too large, the durability to cracking on repeated impact may worsen.

As indicated above, the envelope layer-encased sphere must have a lower surface hardness than the core. The difference between these surface hardnesses (surface hardness of envelope layer-encased sphere—surface hardness of core), expressed on the Shore D scale, is preferably from −30 to −1, more preferably from −25 to −3, and even more preferably from −20 to −5. When this difference is too small, it may not be possible to obtain a feel at impact that is both soft and solid. On the other hand, when this difference is too large, the spin rate may rise excessively on full shots, as a result of which a good distance may not be achieved.

Letting P and Q be the deflections (mm) of, respectively, the core and the ball when each is compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), the value P-Q is preferably from 0.2 to 1.3 mm, more preferably from 0.4 to 1.1 mm, and even more preferably from 0.6 to 0.9 mm. When this value is too large, the initial velocity when the ball is struck may decrease and a good distance may not be obtained. On the other hand, when this value is too small, the spin rate-lowering effect on shots with a driver (W#1) may be inadequate and a good distance may not be obtained.

Thickness Relationship Among Layers

In this invention, the intermediate layer is formed so as to be thicker than the envelope layer and the cover. The value obtained by subtracting the envelope layer thickness from the intermediate layer thickness is preferably from 0.05 to 0.5 mm, more preferably from 0.1 to 0.4 mm, and even more preferably from 0.2 to 0.3 mm. The value obtained by subtracting the cover thickness from the intermediate layer thickness is preferably from 0.01 to 0.3 mm, more preferably from 0.05 to 0.2 mm, and even more preferably from 0.1 to 0.15 mm. In golf balls where the thicknesses of the various layers fall outside of the above ranges, the spin rate-lowering effect on shots with a driver (W#1) may be inadequate and a good distance may not be obtained.

Weight Relationship Among Layers

In this invention, although not particularly limited, it is desirable for the intermediate layer to have a higher weight than the envelope layer and the cover. That is, the value obtained by subtracting the envelope layer weight from the intermediate layer weight is preferably from 0.1 to 2.0 g, more preferably from 0.3 to 1.5 g, and even more preferably from 0.5 to 1.0 g. The value obtained by subtracting the cover weight from the intermediate layer weight is preferably from 0.01 to 0.5 g, more preferably from 0.02 to 0.2 g, and even more preferably from 0.03 to 0.04 g. In golf balls where the weights of the various layers fall outside of the above ranges, the spin rate-lowering effect on shots with a driver (W#1) may be inadequate and a good distance may not be obtained.

Numerous dimples may be formed on the outside surface of the cover serving as the outermost layer. The number of dimples arranged on the cover surface, although not particularly limited, is preferably at least 250, more preferably at least 300, and even more preferably at least 320. The upper limit is preferably not more than 380, more preferably not more than 350, and even more preferably not more than 340. When the number of dimples is higher than this range, the ball trajectory may become lower, as a result of which the distance traveled by the ball may decrease. On the other hand, when the number of dimples is lower that this range, the ball trajectory may become higher, as a result of which a good distance may not be achieved.

The dimple shapes used may be of one type or may be a combination of two or more types suitably selected from among, for example, circular shapes, various polygonal shapes, dewdrop shapes and oval shapes. When circular dimples are used, the dimple diameter may be set to at least about 2.5 mm and up to about 6.5 mm, and the dimple depth may be set to at least 0.08 mm and up to 0.30 mm.

In order for the aerodynamic properties to be fully manifested, it is desirable for the dimple coverage ratio on the spherical surface of the golf ball, i.e., the dimple surface coverage SR, which is the sum of the individual dimple surface areas, each defined by the flat plane circumscribed by the edge of a dimple, as a percentage of the spherical surface area of the ball were the ball to have no dimples thereon, to be set to at least 70% and not more than 90%. Also, to optimize the ball trajectory, it is desirable for the value Vo, defined as the spatial volume of the individual dimples below the flat plane circumscribed by the dimple edge, divided by the volume of the cylinder whose base is the flat plane and whose height is the maximum depth of the dimple from the base, to be set to at least 0.35 and not more than 0.80. Moreover, it is preferable for the ratio VR of the sum of the volumes of the individual dimples, each formed below the flat plane circumscribed by the edge of a dimple, with respect to the volume of the ball sphere were the ball surface to have no dimples thereon, to be set to at least 0.6% and not more than 1.0%. Outside of the above ranges in these respective values, the resulting trajectory may not enable a good distance to be obtained and so the ball may fail to travel a fully satisfactory distance.

In addition, by optimizing the cross-sectional shape of the dimples, the variability in the flight of the ball can be reduced and the aerodynamic performance improved. Moreover, by holding the percentage change in depth at given positions in the dimples within a fixed range, the dimple effect can be stabilized and the aerodynamic performance improved. The ball has arranged thereon at least one dimple with the cross-sectional shape shown below. This is exemplified by dimples having distinctive cross-sectional shapes like that shown in FIG. 3A. FIG. 3A is an enlarged cross-sectional view of a dimple that is circular as seen from above. In this diagram, the symbol D represents a dimple, E represents an edge of the dimple, P represents a deepest point of the dimple, the straight line L is a reference line which passes through the dimple edge E and a center O of the dimple, and the dashed line represents an imaginary spherical surface. The foot of a perpendicular drawn from the deepest point P of the dimple D to an imaginary plane defined by the peripheral edge of the dimple D coincides with the dimple center O. The dimple edge E serves as the boundary between the dimple D and regions (lands) on the ball surface where dimples D are not formed, and corresponds to points where the imaginary spherical surface is tangent to the ball surface (the same applies below). The dimples D shown in FIG. 3 are circular dimples as seen from above; i.e., in a plan view. The center O of the dimple in each plan view coincides with the deepest point P.

The cross-sectional shape of the dimple D must satisfy the following conditions.

First, as condition (i), let the foot of a perpendicular drawn from a deepest point P of the dimple to an imaginary plane defined by a peripheral edge of the dimple be the dimple center O, and let a straight line that passes through the dimple center O and any one point on the edge E of the dimple be the reference line L.

Next, as condition (ii), divide a segment of the reference line L from the dimple edge E to the dimple center O into at least 100 points. Then compute the distance ratio for each point when the distance from the dimple edge E to the dimple center O is set to 100%. The dimple edge E is the origin, which is the 0% position on the reference line L, and the dimple center O is the 100% position with respect to segment EO on the reference line L.

Next, as condition (iii), compute the dimple depth ratio at every 20% from 0 to 100% of the distance from the dimple edge E to the dimple center O. In this case, the dimple center O is at the deepest part P of the dimple and has a depth H (mm). Letting this be 100% of the depth, the dimple depth ratio at each distance is determined. The dimple depth ratio at the dimple edge E is 0%.

Next, as condition (iv), at the depth ratios in dimple regions 20 to 100% of the distance from the dimple edge E to the dimple center O, determine the change in depth ΔH every 20% of the distance and design a dimple cross-sectional shape such that the change ΔH is at least 6% and not more than 24% in all regions corresponding to from 20 to 100% of the distance.

In this invention, by quantifying the cross-sectional shape of the dimple in this way, that is, by setting the change in dimple depth ΔH to at least 6% and not more than 24%, and thereby optimizing the dimple cross-sectional shape, the flight variability decreases, enhancing the aerodynamic performance of the ball. This change ΔH is preferably from 8 to 22%, and more preferably from 10 to 20%.

Also, to further increase the advantageous effects of the invention, in dimples having the above specific cross-sectional shape, it is preferable for the change in dimple depth ΔH to reach a maximum at 20% of the distance from the dimple edge E to the dimple center O. Moreover, it is preferable for two or more points of inflection to be included on the curved line describing the cross-sectional shape of the dimple having the above specific cross-sectional shape.

To ensure a good ball appearance, it is preferable to apply a clear coating onto the cover surface. The coating composition used in clear coating is preferably one which uses two types of polyester polyol as the base resin and uses a polyisocyanate as the curing agent. In this case, various organic solvents can be admixed depending on the intended coating conditions. Examples of organic solvents that can be used include aromatic solvents such as toluene, xylene and ethylbenzene; ester solvents such as ethyl acetate, butyl acetate, propylene glycol methyl ether acetate and propylene glycol methyl ether propionate; ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone; ether solvents such as diethylene glycol dimethyl ether, diethylene glycol diethyl ether and dipropylene glycol dimethyl ether; alicyclic hydrocarbon solvents such as cyclohexane, methyl cyclohexane and ethyl cyclohexane; and petroleum hydrocarbon-based solvents such as mineral spirits.

The coating layer obtained by clear coating has a hardness which, on the Shore C hardness scale, is preferably from 40 to 80, more preferably from 47 to 72, and even more preferably from 55 to 65. If the coating layer is too soft, mud may tend to stick to the surface of the ball when used for golfing. On the other hand, when the coating layer is too hard, it may tend to peel off when the ball is struck. The coating layer has a thickness of typically from 9 to 22 μm, preferably from 11 to 20 μm, and more preferably from 13 to 18 μm.

The multi-piece solid golf ball of the invention can be made to conform to the Rules of Golf for play. The inventive ball may be formed to a diameter which is such that the ball does not pass through a ring having an inner diameter of 42.672 mm and is not more than 42.80 mm, and to a weight which is preferably between 45.0 and 45.93 g.

EXAMPLES

The following Examples and Comparative Examples are provided to illustrate the invention, and are not intended to limit the scope thereof.

Examples 1 to 4, Comparative Examples 1 to 7 Formation of Core

Solid cores were produced by preparing rubber compositions for the respective Working Examples and Comparative Examples shown in Table 1, and then molding and vulcanizing the compositions under vulcanization conditions of 155° C. and 15 minutes.

TABLE 1 Working Example Comparative Example Core formulation (pbw) 1 2 3 4 1 2 3 4 5 6 7 Polybutadiene A 80 80 Polybutadiene B 20 20 20 20 20 20 20 20 20 20 20 Polybutadiene C 80 80 80 80 80 80 80 80 80 Zinc acrylate 37.0 34.9 37.0 34.9 22.4 20.3 37.0 37.0 37.0 37.0 37.0 Organic peroxide (1) 1.0 1.0 1.0 1.0 0.6 0.6 1.0 1.0 1.0 1.0 1.0 Organic peroxide (2) 0.6 0.6 Water 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 Antioxidant 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Barium sulfate 32.2 33.0 Zinc oxide 30.6 31.4 29.4 31.4 4.0 4.0 33.2 32.7 30.7 29.4 29.9 Zinc salt of 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 pentachlorothiophenol

Details on the ingredients mentioned in Table 1 are given below.

-   Polybutadiene A: Available under the trade name “BR 01” from JSR     Corporation -   Polybutadiene B: Available under the trade name “BR 51” from JSR     Corporation -   Polybutadiene C: Available under the trade name “BR 730” from JSR     Corporation -   Zinc acrylate: Available as “ZN-DA85S” from Nippon Shokubai Co.,     Ltd. -   Organic Peroxide (1): Dicumyl peroxide, available under the trade     name “Percumyl D” from NOF Corporation -   Organic Peroxide (2): A mixture of 1,1-di(t-butylperoxy)cyclohexane     and silica, available under the trade name “Perhexa C-40” from NOF     Corporation -   Water: Pure water (from Seiki Chemical Industrial Co., Ltd.) -   Antioxidant: 2,2′-Methylenebis(4-methyl-6-butylphenol), available     under the trade name “Nocrac NS-6” from Ouchi Shinko Chemical     Industry Co., Ltd. -   Barium sulfate: Baryte powder available under the trade name “Barico     #100” from Hakusui Tech -   Zinc oxide: Available under the trade name “Zinc Oxide Grade 3” from     Sakai Chemical Co., Ltd. -   Zinc salt of pentachlorothiophenol: Available from Wako Pure     Chemical Industries, Ltd.

Formation of Envelope Layer and Intermediate Layer

Next, in each Working Example and Comparative Example other than Comparative Example 7, an envelope layer was formed by injection molding the envelope layer material formulated as shown in Table 2 over the core, following which the intermediate layer was formed by injection molding the intermediate layer material formulated as shown in the same table, thereby giving a sphere encased by an envelope layer and an intermediate layer. In Comparative Example 7, an intermediated layer was formed by injection molding the intermediate layer material formulated as shown in Table 2 over the core, thereby giving an intermediate layer-encased sphere.

Formation of Cover (Outermost Layer)

Next, in all of the Working Examples and Comparative Examples, a cover (outermost layer) was formed by injection molding the cover material formulated as shown in Table 2 over the intermediate layer-encased sphere obtained as described above. A plurality of given dimples common to all the Working Examples and Comparative Examples were formed at this time on the surface of the cover.

TABLE 2 Resin composition (pbw) No. 1 No. 2 No. 3 No. 4 No. 5 Hytrel 3001 100 Hytrel 4001 100 Himilan 1557 20 Himilan 1855 30 AM7318 75 AM7327 25 Surlyn 8120 30 Surlyn 9320 70 Nucrel AN4311 20 Nucrel AN4221C 30 Magnesium stearate 60 Kyowamag MF-150 1.12 Titanium oxide 4.0 4.0

Trade names of the chief materials mentioned in the table are given below.

-   Hytrel: Polyester elastomers available from DuPont-Toray Co., Ltd. -   Himilan, AM7318, AM7327: lonomers available from DuPont-Mitsui     Polychemicals Co., Ltd. -   Surlyn: lonomers available from E.I. DuPont de Nemours & Co. -   Nucrel: Ethylene-(meth)acrylate copolymers available from     DuPont-Mitsui Polychemicals Co., Ltd. -   Kyowamag MF-150: Magnesium oxide available from Kyowa Chemical     Industry Co., Ltd.

Dimples

Two families of dimples were used on the ball surface: A and B. Family A includes four types of dimples, details of which are shown in Table 3. The cross-sectional shape of these dimples is shown in FIG. 3A. Family B dimples include four types of dimples, details of which are shown in Table 4. The cross-sectional shape of the latter dimples is shown in FIG. 3B.

In the cross-sectional shapes in FIG. 3, the depth of each dimple from the reference line L to the inside wall of the dimple was determined at 100 equally spaced points on the reference line L from the dimple edge E to the dimple center O. The results are presented in Tables 3 and 4.

Next, the change in depth ΔH every 20% of the distance along the reference line L from the dimple edge E was determined. These values as well are presented in Tables 3 and 4.

TABLE 3 Family A Dimple type No. 1 No. 2 No. 3 No. 4 Number of dimples 240 72 12 14 Diameter (mm) 4.3 3.8 2.8 4.0 Depth at point of maximum depth (mm) 0.15 0.16 0.17 0.16 Dimple depths 20% 0.06 0.07 0.07 0.07 at each point (mm) 40% 0.08 0.09 0.09 0.09 60% 0.11 0.11 0.12 0.11 80% 0.13 0.14 0.15 0.14 100% 0.15 0.16 0.17 0.16 Percent change  0%-20% 41 41 41 41 in dimple depth 20%-40% 15 15 15 15 40%-60% 15 15 15 15 60%-80% 19 19 19 19  80%-100% 10 10 10 10 SR (%) 80 VR (%) 0.9 Percent of dimples having specified shape 100 (%)

TABLE 4 Family B Dimple type No. 1 No. 2 No. 3 No. 4 Number of dimples 240 72 12 14 Diameter (mm) 4.3 3.8 2.8 4.0 Depth at point of maximum depth (mm) 0.14 0.15 0.15 0.16 Dimple depths 20% 0.05 0.05 0.06 0.06 at each point (mm) 40% 0.09 0.10 0.10 0.11 60% 0.12 0.13 0.13 0.13 80% 0.14 0.14 0.14 0.15 100%  0.14 0.15 0.15 0.16 Percent change  0%-20% 35 37 37 38 in dimple depth 20%-40% 30 33 31 29 40%-60% 21 17 18 17 60%-80% 11 10 10 11  80%-100% 4 4 3 5 SR (%) 79 VR (%) 0.9 Percent of dimples having specified shape 0 (%)

Formation of Paint Film Layer (Coating Layer)

Next, the paint formulated as shown in Table 5 below was applied with an air spray gun onto the surface of the cover (outermost layer) on which numerous dimples had been formed, thereby producing golf balls having a 15 m-thick paint film layer formed thereon.

TABLE 5 Paint formulation (pbw) Base resin Polyol 29.77 Additive 0.22 Solvent 70.01 Curing agent Isocyanate 42 Solvent 58 Paint film properties Shore C hardness 62.5 Thickness (μm) 15

A polyester polyol synthesized as follows was used as the polyol in the base resin.

A reactor equipped with a reflux condenser, a dropping funnel, a gas inlet and a thermometer was charged with 140 parts by weight of trimethylolpropane, 95 parts by weight of ethylene glycol, 157 parts by weight of adipic acid and 58 parts by weight of 1,4-cyclohexanedimethanol, following which the temperature was raised to between 200 and 240° C. under stirring and the reaction was effected by 5 hours of heating. This yielded a polyester polyol having an acid value of 4, a hydroxyl value of 170 and a weight-average molecular weight (Mw) of 28,000. The additives were water repellent additives. Specifically, they were all commercially available silicone-based stain-resistance-improving additives. Fluoropolymers having an alkyl group chain length of 7 or less as a carbon number were added.

The isocyanate used in the curing agent was Duranate™ TPA-100 (from Asahi Kasei Corporation; NCO content, 23.1%; 100% nonvolatiles), an isocyanurate of hexamethylene diisocyanate (HMDI).

Butyl acetate was used as the base resin solvent, and ethyl acetate and butyl acetate were used as the curing agent solvents. The Shore C hardness values in the table were obtained by preparing sheets having a thickness of 2 mm and carrying out measurement with a Shore C durometer in general accordance with ASTM D2240.

Various properties of the resulting golf balls, including the internal hardnesses of the core at various positions, the diameters of the core and the respective layer-encased spheres, the thickness and material hardness of each layer, and the surface hardness and deformation (deflection) under specific loading of the respective layer-encased spheres were evaluated by the following methods. The results are presented in Table 6.

Diameters of Core, Envelope Layer-Encased Sphere and Intermediate Layer-Encased Sphere

The diameters at five random places on the surface were measured at a temperature of 23.9±1° C. and, using the average of these measurements as the measured value for a single core, envelope layer-encased sphere or intermediate layer-encased sphere, the average diameters for ten test specimens were determined.

Diameter of Ball

The diameters at 15 random dimple-free areas on the surface of a ball were measured at a temperature of 23.9±1° C. and, using the average of these measurements as the measured value for a single ball, the average diameter for ten measured balls was determined.

Deflections of Core, Envelope Layer-Encased Sphere, Intermediate Layer-Encased Sphere and Ball

A core, envelope layer-encased sphere, intermediate layer-encased sphere or ball was placed on a hard plate and the amount of deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) was measured. The amount of deflection here refers in each case to the measured value obtained after holding the test specimen isothermally at 23.9° C.

Core Hardness Profile

The indenter of a durometer was set substantially perpendicular to the spherical surface of the core, and the surface hardness of the core on the Shore C hardness scale was measured in accordance with ASTM D2240. Cross-sectional hardnesses at the center of the core and at given positions in each core were measured by perpendicularly pressing the indenter of a durometer against the region to be measured in the flat cross-sectional plane obtained by cutting the core into hemispheres. The measurement results are indicated as Shore C hardness values.

In addition, letting Cc be the Shore C hardness at the core center, Cs be the Shore C hardness at the core surface, C_(M) be the Shore C hardness at a midpoint M between the core center and surface, C_(M+2.5), C_(M+5.0) and C_(M+7.5) be the Shore C hardnesses at, respectively, positions 2.5 mm, 5.0 mm and 7.5 mm from the midpoint M toward the core surface side, and C_(M−2.5), C_(M−5.0) and C_(M−7.5) be the Shore C hardnesses at, respectively, positions 2.5 mm, 5.0 mm and 7.5 mm from the midpoint M toward the core center side, the surface areas A to F defined as follows

-   -   surface area A: ½×2.5×(C_(M−5.0)−C_(M−7.5)),     -   surface area B: ½×2.5×(C_(M−2.5)−C_(M−5.0)),     -   surface area C: ½×2.5×(C_(M)−C_(M−2.5)),     -   surface area D: ½×2.5×(C_(M+2.5)−C_(M)),     -   surface area E: ½×2.5×(C_(M+5.0)−C_(M+2.5)), and     -   surface area F: ½×2.5×(C_(M+7.5)−C_(M+5.0))         were calculated, and the values of the following three         expressions were determined:

(surface area D+surface area E+surface area F)−(surface area A+surface area B+surface area C)

(surface area D+surface area E)−(surface area A+surface area B+surface area C)

[(surface area D+surface area E+surface area F)−(surface area A+surface area B+surface area C)]/(Cs−Cc)

Surface areas A to F in the core hardness distribution are explained in FIG. 2, which is a graph that illustrates surface areas A to F using the core hardness profile data from Working Example 1.

Material Hardnesses (Shore D Hardnesses) of Envelope Layer, Intermediate Layer and Cover

The resin materials for each of these layers were molded into sheets having a thickness of 2 mm and left to stand for at least two weeks, following which the Shore D hardnesses were measured in accordance with ASTM D2240.

Surface Hardnesses (Shore D Hardnesses) of Envelope Layer-Encased Sphere, Intermediate Layer-Encased Sphere and Ball

Measurements were taken by pressing the durometer indenter perpendicularly against the surface of each sphere. The surface hardness of the ball (cover) is the measured value obtained at dimple-free places (lands) on the ball surface. The Shore D hardnesses were measured with a type D durometer in accordance with ASTM D2240.

TABLE 6 Working Example Comparative Example 1 2 3 4 1 2 3 4 5 6 7 Construction 4-piece 4-piece 4-piece 4-piece 4-piece 4-piece 4-piece 4-piece 4-piece 4-piece 3-piece Core Diameter (mm) 36.1 36.1 36.1 36.1 36.1 36.1 35.2 35.2 36.1 36.1 37.3 Weight (g) 29.5 29.5 29.4 29.5 29.5 29.5 27.7 27.6 29.5 29.4 32.5 Specific gravity (g/mm³) 1.199 1.199 1.192 1.199 1.199 1.199 1.213 1.210 1.199 1.192 1.195 Deflection (P) (mm) 4.3 4.7 4.3 4.7 4.3 4.7 4.3 4.3 4.3 4.3 4.3 Surface hardness (Cs) 81 77 81 77 76 72 80 80 81 81 82 Core Hardness 15 mm from center (C15) 77 73 77 73 71 67 77 77 77 77 77 hardness Hardness 12.5 mm from center (C12.5) 71 68 71 68 71 67 71 71 71 71 71 profile Hardness 10 mm from center (C10) 64 63 64 63 68 64 64 64 64 64 64 Hardness 7.5 mm from center (C7.5) 61 60 61 60 66 62 61 61 61 61 61 Hardness 5 mm from center (C5) 59 58 59 58 65 61 59 59 59 59 59 Hardness 2.5 mm from center (C2.5) 56 56 56 56 62 58 56 56 56 56 56 Center hardness (Cc) 54 52 54 52 60 56 54 54 54 54 54 Hardness 7.5 mm toward core surface 79 75 79 75 73 69 78 78 79 79 79 side from midpoint M (C_(M+7.5)) Hardness 5 mm toward core surface 74 71 74 71 71 67 74 74 74 74 74 side from midpoint M (C_(M+5)) Hardness 2.5 mm toward core surface 68 65 68 65 69 66 68 68 68 68 68 side from midpoint M (C_(M+2.5)) Hardness at midpoint M (C_(M)) 63 62 63 62 67 63 63 63 63 63 63 Hardness 2.5 mm toward core center 60 59 60 59 66 62 60 60 60 60 60 side from midpoint M (C_(M−2.5)) Hardness 5 mm toward core center 58 57 58 57 64 60 58 58 58 58 58 side from midpoint M (C_(M−5)) Hardness 7.5 mm toward core center 55 54 55 54 61 57 55 55 55 55 55 side from midpoint M (C_(M−7.5)) Surface hardness − 27 25 27 25 16 16 26 26 27 27 28 Center hardness (Cs − Cc) Surface area A: ½ × 2.5 × (C_(M−5) − C_(M−7.5)) 3.3 3.8 3.3 3.8 3.8 3.8 3.3 3.3 3.3 3.3 3.3 Surface area B: ½ × 2.5 × (C_(M−2.5) − C_(M−5)) 2.9 2.5 2.9 2.5 2.5 2.5 2.9 2.9 2.9 2.9 2.9 Surface area C: ½ × 2.5 × (C_(M) − C_(M−2.5)) 3.8 3.8 3.8 3.8 1.3 1.3 3.8 3.8 3.8 3.8 3.8 Surface area D: ½ × 2.5 × (C_(M+2.5) − C_(M)) 6.3 3.8 6.3 3.8 2.5 3.8 6.3 6.3 6.3 6.3 6.3 Surface area E: ½ × 2.5 × (C_(M+5) − C_(M+2.5)) 7.5 7.5 7.5 7.5 2.5 1.3 7.5 7.5 7.5 7.5 7.5 Surface area F: ½ × 2.5 × (C_(M+7.5) − C_(M+5)) 6.3 5 6.3 5 2.5 2.5 5 5 6.3 6.3 6.3 Surface areas A + B + C 10 10 10 10 7.5 7.5 10 10 10 10 10 Surface areas D + E 13.8 11.3 13.8 11.3 5 5 13.8 13.8 13.8 13.8 13.8 Surface areas D + E + F 20 16.3 20 16.3 7.5 7.5 18.8 18.8 20 20 20 (Surface areas D + E + F) − 10 6.3 10 6.3 0 0 8.8 8.8 10 10 10 (Surface areas A + B + C) (Surface areas D + E) − 3.8 1.3 3.8 1.3 −2.5 −2.5 3.8 3.8 3.8 3.8 3.8 (Surface areas A + B + C) [(Surface areas D + E + F) − 0.37 0.25 0.37 0.25 0 0 0.34 0.34 0.37 0.37 0.36 (Surface areas A + B + C)]/(Cs − Cc) Surface hardness (Shore D) 54 51 54 51 50 47 53 53 54 54 54 Envelope Material No. 1 No. 1 No. 2 No. 1 No. 1 No. 1 No. 1 No. 1 No. 1 No. 3 — layer Thickness (mm) 1 1 1 1 1 1 1.2 1.4 1 1 — Specific gravity (g/mm³) 1.08 1.08 1.12 1.08 1.08 1.08 1.08 1.08 1.08 0.964 — Weight (g) 4.67 4.67 4.84 4.67 4.67 4.67 5.4 6.12 4.67 4.17 — Material hardness (sheet hardness: Shore D) 27 27 40 27 27 27 27 27 27 52 — Envelope Diameter (mm) 38.1 38.1 38.1 38.1 38.1 38.1 37.6 37.9 38.1 38.1 — layer- Weight (g) 34.2 34.2 34.2 34.2 34.2 34.2 33.1 33.8 34.2 33.5 — encased Deflection (mm) 4.2 4.6 4.2 4.6 4.1 4.6 4 4 4.2 4 — sphere Surface hardness (Shore D) 35 35 48 35 35 35 33 33 35 35 — Surface hardness of envelope layer-encased sphere − −19 −16 −6 −16 −15 −12 −20 −20 −19 −19 — Core surface hardness (Shore D) Intermediate Material No. 3 No. 3 No. 3 No. 3 No. 3 No. 3 No. 3 No. 3 No. 3 No. 1 No. 3 layer Thickness (mm) 1.2 1.2 1.2 1.2 1.2 1.2 1.25 1.25 1.2 1.2 1.4 Specific gravity (g/mm³) 0.964 0.964 0.964 0.964 0.964 0.964 0.964 0.964 0.964 1.08 0.964 Weight (g) 5.61 5.61 5.61 5.61 5.61 5.61 5.72 5.8 5.61 6.29 6.35 Material hardness (sheet hardness: Shore D) 52 52 52 52 52 52 52 52 52 52 52 Intermediate Diameter (mm) 40.5 40.5 40.5 40.5 40.5 40.5 40.1 40.4 40.5 40.5 40.1 layer- Weight (g) 39.8 39.8 39.8 39.8 39.8 39.8 38.8 39.6 39.8 39.8 38.8 encased Deflection (mm) 3.9 4.2 3.9 4.2 3.8 4.2 3.6 3.6 3.9 3.9 4 sphere Surface hardness (Shore D) 58 58 58 58 58 58 58 58 58 33 58 Surface hardness of intermediate layer-encased sphere − 23 23 10 23 23 23 25 25 23 −2 — Surface hardness of envelope layer-encased sphere (Shore D) Cover Material No. 4 No. 4 No. 4 No. 4 No. 4 No. 4 No. 4 No. 4 No. 5 No. 4 No. 4 Thickness (mm) 1.1 1.1 1.1 1.1 1.1 1.1 1.3 1.15 1.1 1.1 1.3 Specific gravity (g/mm³) 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 Weight (g) 5.58 5.58 5.58 5.58 5.58 5.58 6.58 5.83 5.58 5.58 6.58 Material hardness (sheet hardness: Shore D) 62 62 62 62 62 62 62 62 50 62 56 Coating layer Paint C Paint C Paint C Paint C Paint C Paint C Paint C Paint C Paint C Paint C Paint C Ball Diameter (mm) 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 Weight (g) 45.5 45.5 45.5 45.5 45.5 45.5 45.5 45.5 45.5 45.5 45.5 Deflection (Q) (mm) 3.6 3.9 3.6 3.9 3.5 3.9 3.2 3.3 3.7 3.6 3.5 Surface hardness (Shore D) 68 68 68 68 68 68 68 68 56 68 62 Dimples Family A Family A Family A Family B Family A Family A Family A Family A Family A Family A Family A Ball surface hardness − Surface hardness of 10 10 10 10 10 10 10 10 −2 35 4 intermediate layer-encased sphere (Shore D) Ball surface hardness − Core surface hardness (Shore D) 14 17 14 17 18 21 15 15 2 14 8 Intermediate layer thickness − Cover thickness (mm) 0.1 0.1 0.1 0.1 0.1 0.1 −0.05 0.1 0.1 0.1 0.1 Intermediate layer thickness − Envelope layer thickness (mm) 0.2 0.2 0.2 0.2 0.2 0.2 0.05 −0.1 0.2 0.2 — Intermediate layer weight − Envelope layer weight (g) 0.94 0.94 0.77 0.94 0.94 0.94 0.32 −0.32 0.94 2.12 — Intermediate layer weight − Cover weight (g) 0.03 0.03 0.03 0.03 0.03 0.03 −0.87 −0.03 0.03 0.71 −0.23 Difference in deflection (P − Q) (mm) 0.7 0.8 0.7 0.8 0.8 0.8 1.1 1.0 0.6 0.7 0.8

The flight performance (W#1) and feel of each golf ball were evaluated by the following methods. The results are shown in Table 7.

Flight Performance

A driver (W#1) was mounted on a golf swing robot and the distance traveled by the ball when struck at a head speed of 35 m/s was measured and rated according to the criteria shown below. The club used was the PHYZ Driver (loft angle, 10.5°) manufactured by Bridgestone Sports Co., Ltd. In addition, using an apparatus for measuring the initial conditions, the spin rate was measured immediately after the ball was similarly struck.

Rating Criteria

-   -   Good: Total distance was 177.0 m or more     -   NG: Total distance was less than 177.0 m

Feel

Sensory evaluations were carried out when the balls were hit with a driver (W#1) by amateur golfers having head speeds of 30 to 40 m/s. The feel of the ball was rated according to the following criteria.

Rating Criteria:

-   -   Excellent (Exc): Eight or more of ten golfers rated the feel as         good     -   Good: Six or seven of ten golfers rated the feel as good     -   NG: Five or fewer of ten golfers rated the feel as good

TABLE 7 Working Example Comparative Example 1 2 3 4 1 2 3 4 5 6 7 Flight Spin rate (rpm) 2,914 2,805 2,867 2,815 2,999 2,855 3,038 3,048 3,003 2,944 2,850 (W#1) HS, Total distance (m) 177.3 178.7 177.7 178.2 176.2 176.8 175.1 175.7 173.9 176.5 178.9 35 m/s Rating Good Good Good Good NG NG NG NG NG NG Good Feel Rating Exc Exc Good Exc Exc Exc Exc Exc Exc Exc NG

As demonstrated by the results in Table 7, the golf balls of Comparative Examples 1 to 7 were inferior in the following respects to the golf balls according to the present invention that were obtained in the Working Examples.

In Comparative Example 1, the core hardness profile did not satisfy the condition (surface areas D+E+F)−(surface areas A+B+C)>0, as a result of which the spin rate of the ball increased, resulting in a poor distance.

In Comparative Example 2, the core hardness profile did not satisfy the condition (surface areas D+E+F)−(surface areas A+B+C)>0, as a result of which the spin rate of the ball increased, resulting in a poor distance.

In Comparative Example 3, the cover was thicker than the intermediate layer, as a result of which the spin rate of the ball increased, resulting in a poor distance.

In Comparative Example 4, the envelope layer was thicker than the intermediate layer, as a result of which the spin rate of the ball increased, resulting in a poor distance.

In Comparative Example 5, the surface hardness of the ball was lower than the surface hardness of the intermediate layer-encased sphere, as a result of which the spin rate of the ball increased and the initial velocity of the ball when struck was low, resulting in a poor distance.

In Comparative Example 6, the surface hardness of the envelope layer-encased sphere was lower than the surface hardness of the intermediate layer-encased sphere, as a result of which the spin rate of the ball increased, resulting in a poor distance.

The ball in Comparative Example 7 was a three-piece ball that was not provided with a soft envelope layer. As a result, the feel of the ball at impact was poor.

Japanese Patent Application No. 2018-094643 is incorporated herein by reference.

Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims. 

1. A multi-piece solid golf ball comprising a core, an envelope layer, an intermediate layer and a cover, wherein the core is formed primarily of a base rubber and the intermediate layer is formed so as to be thicker than the envelope layer and the cover; the core has a surface hardness, the sphere obtained by encasing the core with the envelope layer (envelope layer-encased sphere) has a surface hardness, the sphere obtained by encasing the envelope layer-encased sphere with the intermediate layer (intermediate layer-encased sphere) has a surface hardness and the ball has a surface hardness which together satisfy to the following relationship: surface hardness of ball >surface hardness of intermediate layer-encased sphere >surface hardness of envelope layer-encased sphere <surface hardness of core; and the core has a hardness profile in which, letting Cc be the Shore C hardness at a center of the core, Cs be the Shore C hardness at the core surface, C_(M) be the Shore C hardness at a midpoint M between the core center and surface, C_(M+2.5), C_(M+5.0) and C_(M+7.5) be the Shore C hardnesses at, respectively, positions 2.5 mm, 5.0 mm and 7.5 mm from the midpoint M toward the core surface side, and C_(M−2.5), C_(M−5.0) and C_(M−7.5) be the Shore C hardnesses at, respectively, positions 2.5 mm, 5.0 mm and 7.5 mm from the midpoint M toward the core center side, the surface areas A to F defined as follows surface area A: ½×2.5×(C_(M−5.0)−C_(M−7.5)), surface area B: ½×2.5×(C_(M−2.5)−C_(M−5.0)), surface area C: ½×2.5×(C_(M)−C_(M−2.5)), surface area D: ½×2.5×(C_(M+2.5)−C_(M)), surface area E: ½×2.5×(C_(M+5.0)−C_(M+2.5)), surface area F: ½×2.5×(C_(M+7.5)−C_(M+5.0)), satisfy the condition (surface area D+surface area E+surface area F)−(surface area A+surface area B+surface area C)>0.
 2. The golf ball of claim 1, wherein the surface areas A to F in the core hardness profile satisfy the condition (surface area D+surface area E)−(surface area A+surface area B+surface area C)≥0.
 3. The golf ball of claim 1, wherein the hardness difference between the surface and center of the core (Cs−Cc), expressed in terms of Shore C hardness, is at least
 20. 4. The golf ball of claim 1, wherein the surface areas A to F in the core hardness profile satisfy the condition 0.15≤[(surface area D−surface area E+surface area F)−(surface area A+surface area B+surface area C)]/(Cs−Cc)≤0.6.
 5. The golf ball of claim 1, wherein the cover has a thickness of 1.2 mm or less.
 6. The golf ball of claim 1, wherein the cover has a plurality of dimples formed on a surface thereof, the ball has arranged thereon at least one dimple with a cross-sectional shape that is described by a curved line or a combination of straight and curved lines and specified by steps (i) to (iv) below, and the total number of dimples is from 250 to 380: (i) letting the foot of a perpendicular drawn from a deepest point of the dimple to an imaginary plane defined by a peripheral edge of the dimple be the dimple center and a straight line that passes through the dimple center and any one point on the edge of the dimple be the reference line: (ii) dividing a segment of the reference line from the dimple edge to the dimple center into at least 100 points and computing the distance ratio for each point when the distance from the dimple edge to the dimple center is set to 100%; (iii) computing the dimple depth ratio at every 20% from 0 to 100% of the distance from the dimple edge to the dimple center; and (iv) at the depth ratios in dimple regions 20 to 100% of the distance from the dimple edge to the dimple center, determining the change in depth ΔH every 20% of said distance and designing a dimple cross-sectional shape such that the change ΔH is at least 6% and not more than 24% in all regions corresponding to from 20 to 100% of said distance.
 7. The golf ball of claim 1, wherein the cover has a paint film layer formed on a surface thereof, which paint film layer has a hardness on the Shore C scale of from 40 to
 80. 